1. Field of the Invention
The present invention relates to steel particularly suited for both general use and the manufacture of a component for bearings, especially a race for ball bearings, needle bearings or roller bearings.
2. Background of the Invention
Components for bearings, such as races, balls, needles or rollers, are generally manufactured from steel of the 100Cr6 or 100CrMn6 type containing from 0.6 to 1.5% of carbon, from 1.3 to 1.6% of chromium, from 0.3 to 1% of manganese and less than 0.4% of silicon and having a very high degree of cleanliness in terms of inclusions. The steel is used in the form of rolled bar, seamless tube or wire, from which are cut blanks or slugs which are formed by cold or hot plastic deformation and then hardened by tempering and annealing, before being machined. The components thus obtained have a high hardness and the toughness required for them to be able to withstand the rolling fatigue well, at least under the normal conditions of use, especially for in-service temperatures below 150xc2x0 C. However, the components thus formed have an insufficient rolling fatigue resistance for more severe service conditions, which are tending to become common. These more severe service conditions are characterized, in particular, by a service temperature above 150xc2x0 C. and possibly as high as 350xc2x0 C., and/or by the presence of the phenomenon of bearing surface deterioration by indentation. This phenomenon consists of the initiation of cracks on the surface, caused by the indentations, i.e. deformations generated by hard particles present in the lubricant.
In order to limit the effect of the indentation, it has been proposed to use materials having a very high hardness such as ceramics or deposits of hard materials. However, this technique has the drawback of being not very reliable because of the excessively high brittleness of these materials, which brittleness makes them very sensitive to the slightest defect.
It has also been proposed, for example in U.S. Pat. No. 5,030,017, to use a steel containing, in particular, from 0.3% to 0.6% of carbon, from 3% to 14% of chromium, from 0.4% to 2% of molybdenum, from 0.3% to 1% of vanadium and less than 2% of manganese. The components are carburized or carbonitrided in the region of the bearing surface, so as to obtain a sum of the carbon and nitrogen contents of between 0.03% and 1%, and are then hardened so that their micrographic structure comprises from 20% to 50% (in % by volume) of residual austenite in a surface layer representing from 10% to 25% of the volume of the component. This technique has the double drawback of requiring the use of a steel which is highly loaded with alloying elements, and is hence expensive, and the execution of a carburizing or carbonitriding treatment, this treatment being lengthy and expensive.
It has also been proposed, in German Patent Application DE 195 24 957, to use a steel containing from 0.9% to 1.3% of carbon, from 0.6% to 1.2% of silicon, from 1.1% to 1.6% of manganese and from 1.3% to 1.7% of chromium, the balance being iron and impurities resulting from smelting, the structure of this steel containing from 7% to 25% of residual austenite. However, this steel, because of its chemical composition, provides no guarantee of castability; of cold deformability and of residual austenite content and stability. The specified residual austenite content necessary for improving the resistance to indentation fatigue moreover requires, in the case of this steel, subjecting the bearings to a not very convenient heat treatment comprising a step of holding them at approximately 100xc2x0 C. for more than 10 hours between tempering and annealing without returning to ambient temperature after tempering or before annealing. Moreover, in the presence of multidirectional stresses below the cyclic yield stress, its austenite is stable for more than 2000 hours only for thermal stresses below 120xc2x0 C., which is too low for some applications.
The object of the present invention is to remedy these drawbacks by providing a means for manufacturing, in an economical manner and especially using a relatively standard heat treatment, a component for bearings which is resistant to indentation, in particular when they are used briefly or occasionally above 300xc2x0 C., and is not very brittle.
For this purpose, the subject of the invention is a steel useful generaly and for the manufacture of a component for bearings, the chemical composition of which comprises, by weight:
0.6%xe2x89xa6Cxe2x89xa61.5%
0.4%xe2x89xa6Mnxe2x89xa61.5%
0.75%xe2x89xa6Sixe2x89xa62.5%
0.2%xe2x89xa6Crxe2x89xa62%
0%xe2x89xa6Nixe2x89xa60.5%
0%xe2x89xa6Moxe2x89xa60.2%
0% less than Alxe2x89xa60.05%
0% less than Tixe2x89xa60.04%
Sxe2x89xa60.04%
the balance being iron and impurities resulting from smelting, the composition furthermore satisfying the relationships:
Mnxe2x89xa60.75+0.55xc3x97Si
Mnxe2x89xa62.5xe2x88x920.8xc3x97Si
Preferably, the chemical composition of the steel is such that, separately or better still at the same time, on the one hand:
0.8%xe2x89xa6Mnxe2x89xa61.2%
0.8%xe2x89xa6Sixe2x89xa61.7%
and, on the other hand:
0.9%xe2x89xa6Cxe2x89xa61.1%
1.3%xe2x89xa6Crxe2x89xa61.6%
Also preferably, the silicon content is greater than 1.2%; the inventors have, in fact, surprisingly observed that, when simultaneously the silicon content is greater than 1.2% and the manganese content is less than 1.5%, and preferably less than 1.2% but greater than 0.8%, the stability of the austenite is very substantially improved.
The invention also relates to a process for the manufacture of a component for bearings, in which:
a semi-finished product made of steel according to the invention is provided;
the semi-finished product is formed by hot plastic deformation so as to obtain a product blank and, more particularly, a seamless-tube blank
a spheroidizing treatment is carried out on the product blank, this treatment comprising heating to a temperature of between 750xc2x0 C. and 850xc2x0 C. followed by cooling, the maximum rate of which is 10xc2x0 C./hour, down to 650xc2x0 C. so as to obtain a structure having a hardness of less than 270 HV and comprising a fine dispersion of carbides, and optionally, forming by cold plastic deformation, for example cold rolling or cold drawing so as to obtain a product;
the product is cut in order to obtain a section which is formed by cold or hot plastic deformation, or by machining, so as to obtain a component blank for bearings; and
the component blank for bearings is subjected to a hardening heat treatment by cooling, for example in oil, after austenization between 800xc2x0 C. and 950xc2x0 C. and to an annealing heat treatment between 100xc2x0 C. and 400xc2x0 C. and preferably below 250xc2x0 C., so as to obtain a component for bearings which has a structure whose hardness is between 58 HRC and 67 HRC and which consists of residual carbides, martensite and from 5% to 30% of residual austenite.
Finally, the invention relates, on the one hand, to a seamless tube made of steel according to the invention and, on the other hand, to a component for bearings made of steel according to the invention having a structure consisting of residual carbides, martensite and from 5% to 30% of residual austenite thermally stable up to 400xc2x0 C. at least.